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2 Candidates should be able to:(a) describe the meaning of enthalpy change in terms of exothermic (ΔH negative) andendothermic (ΔH positive) reactions(b) *represent energy changes by energy profile diagrams, including reaction enthalpychanges and activation energies (see 6.1(c))(c) describe bond breaking as an endothermic process and bond making as anexothermic process(d) *explain overall enthalpy changes in terms of the energy changes associated withthe breaking and making of covalent bonds(e) describe combustion of fuels as exothermic, e.g. wood; coal; oil; natural gas;hydrogen(f) describe hydrogen, derived from water or hydrocarbons, as a potential fuel for usein future, reacting with oxygen to generate electricity directly in a fuel cell (details of the constructionand operation of a fuel cell are not required)(g) name natural gas, mainly methane, and petroleum as sources of energy

3 THE CONCEPT OF ENTHALPY CHANGEIn chemical reactions, we can measure changes in terms of energy.We cannot measure the total amount of energy in the beginning, we also cannot measure the total amount of energy in the end.We can measure the change in energy, we call this change the enthalpy change (or heat of reaction).

6 Exothermic Reaction time Reactants give out energy to the surroundingstemperature32C25CtimeReactants give out energy to the surroundings

7 Exothermic Reaction Reactants give out energy to the surroundingsTemperature of surroundings increaseEnergy of reactants decreaseEnergyofreactantsTemperature(surroundings)Products32CReactantsΔH is negative25C

8 Energy Levels Exothermic reactions Energy is given outThe products have less energy than the reactantsCombustion and neutralisation are exothermic

15 Endothermic Reaction timetemperature25C19CtimeReactants take in energy from the surroundings

16 Endothermic Reaction Reactants take in energy from the surroundingsTemperature of surroundings decreaseEnergy of reactants increasesEnergyofreactantsTemperature(surroundings)25CProductsΔH is positiveReactants19C

17 Energy Level Diagrams Endothermic reactions Energy is taken inThe products have more energy than the reactantsThe energy is taken in from the surroundings

23 Endothermic reactionsSummary TableExothermic reactionsEndothermic reactionsEnergy is given out to the surroundingsEnergy is taken in from the surroundings∆H is negative∆H is positiveProducts have less energy than reactantsProducts have more energy than reactants

25 ∆H How much energy is given out or taken in?Energy is needed to break chemical bondsEnergy is given out when bonds are made∆H is the difference between the energy needed to break the bonds in the reactants, and the energy given out when new bonds are made in the products

28 Working out ∆H Summary The energy values have units of kJ/moleIn the exam, you will be given the energy values and all the bonds to make or breakEnergy goes in to break bondsEnergy goes out when bonds are made